Philippe Hervé Leloup

The Ailao Shan-Red River shear zone (Yunnan, China), Tertiary transform boundary of Indochina

The Red River Fault zone (RRF) is the major geological discontinuity that separates South China from Indochina. Today it corresponds to a great right-lateral fault, following for over 900 km the edges of four narrow (< 20 km wide) high-grade gneiss ranges that together form the Ailao Shan-Red River (ASRR) metamorphic belt: the Day Nui Con Voi in Vietnam, and the Ailao, Diancang and Xuelong Shan in Yunnan. The Ailao Shan, the longest of those ranges, is fringed to the south by a strip of low-grade schists that contain ultramafic bodies. The ASRR belt has thus commonly been viewed as a suture. A detailed study of the Ailao and Diancang Shan shows that the gneiss cores of the ranges are composed of strongly foliated and lineated mylonitic gneisses. The foliation is usually steep and the lineation nearly horizontal, both being almost parallel to the local trend of the gneissic cores. Numerous shear criteria, including asymmetric tails on porphyroclasts, C-S or C′-S structures, rolling structures, asymmetric foliation boudinage and asymmetric quartz 〈c〉 axis fabrics, indicate that the gneisses have undergone intense, progressive left-lateral shear. P-T studies show that left-lateral strain occurred under amphibolite-facies conditions (3–7 kb and 550–780°C). In both ranges high-temperature shear was coeval with emplacement of leucocratic melts. Such deformed melts yield UPb ages between 22.4 and 26.3 Ma in the Ailao Shan and between 22.4 and 24.2 Ma in the Diancang Shan, implying shear in the Lower Miocene. The mylonites in either range rapidly cooled to ≈ 300°C between 22 and 17 Ma, before the end of left-lateral motion. The similarity of deformation kinematics, P-T conditions, and crystallization ages in the aligned Ailao and Diancang Shan metamorphic cores, indicate that they represent two segments of the same Tertiary shear zone, the Ailao Shan-Red River (ASRR) shear zone. Our results thus confirm the idea that the ASRR belt was the site of major left-lateral motion, as Indochina was extruded toward the SE as a result of the India-Asia collision. The absence of metamorphic rocks within the 80 km long “Midu gap” between the gneissic cores of the two ranges results from sinistral dismemberment of the shear zone by large-scale boudinage followed by uplift and dextral offset of parts of that zone along the Quaternary Red River Fault. Additional field evidence suggests that the Xuelong Shan in northern Yunnan and the Day Nui Con Voi in Vietnam are the northward and southward extensions, respectively, of the ASRR shear zone, which therefore reaches a length of nearly 1000 km. Surface balance restoration of amphibolite boudins trails indicates layer parallel extension of more than 800% at places where strain can be measured, suggesting shear strains on the order of 30, compatible with a minimum offset of 300 km along the ASRR zone. Various geological markers have been sinistrally offset 500–1150 km by the shear zone. The seafloor-spreading kinematics in the South China Sea are consistent with that sea having formed as a pull apart basin at the southeast end of the ASRR zone, which yields a minimum left-lateral offset of 540 km on that zone. Comparison of Cretaceous magnetic poles for Indochina and South China suggests up to 1200 ± 500 km of left-lateral motion between them. Such concurrent evidence implies a Tertiary finite offset on the order of 700 ± 200 km on the ASRR zone, to which several tens of kilometers of post-Miocene right-lateral offset should probably be added. These results significantly improve our quantitative understanding of the finite deformation of Asia under the thrust of the Indian collision. While being consistent with a two-stage extrusion model, they demonstrate that the great geological discontinuity that separates Indochina from China results from Cenozoic strike-slip strain rather than more ancient suturing. Furthermore, they suggest that this narrow zone acted like a continental transform plate boundary in the Oligo-Miocene, governing much of the motion and tectonics of adjacent regions. 700 and 200 km of left-lateral offset on the ASRR shear zone and Wang Chao fault zone, respectively, would imply that the extrusion of Indochina alone accounted for 10–25% of the total shortening of the Asian continent. The geological youth and degree of exhumation of the ASRR zone make it a worldwide reference model for large-scale, high-temperature, strike-slip shear in the middle and lower crust. It is fair to say that this zone is to continental strike-slip faults what the Himalayas are to mountain ranges.

Intraplate tectonics in Asia: A precise age for large-scale Miocene movement along the Ailao Shan-Red River shear zone, China

Tertiary left-lateral movement along the 1000 km long Ailao Shan-Red River shear zone appears to have played an important role in absorbing post-collisional northward penetration of India into Asia. Crustal strike-slip shear along this zone caused the formation of a gneiss belt, metamorphosed to amphibolite grade, including anatectic melting. Both metamorphism and melting were induced by ductile deformation yielding the possibility to date the major tectonometamorphic event that shaped the Ailao Shan-Red River belt. 17 U-Pb isotope analyses were performed on small size-fractions of zircon, monazite and xenotime, extracted from two different leucogranitic layers. The two samples are located about 50 km apart in the central segment of Ailao Shan, in structurally well controlled settings where the melts crystallized within the strongly foliated gneisses during late stages of deformation. All mineral U-Pb analyses had to be corrected for excess or deficit amounts of radiogenic 206 Pb, originating from initial 230 Th disequilibrium in the 238 U decay series. In monazite, such disequilibrium 206 Pb reaches 20% of total radiogenic 206 Pb. The corrected U-Pb ages of monazite and xenotime lie between 22.1 and 23.9 Ma, whereas zircon yields significantly discordant U-Pb ages between 30.5 and 33.9 Ma pointing to Precambrian material in the magma source region. Inherited components could also be detected in monazite. The set of U-Pb data shows that monazite and xenotime formed simultaneously in both localities substantiating an early Miocene age of 23.0 ± 0.2Ma for late kinematic crystallization of anatectic melts in the metamorphic belt of the Ailao Shan-Red River shear zone.

Large river offsets and Plio‐Quaternary dextral slip rate on the Red River fault (Yunnan, China)

Using multispectral SPOT images and 1/100,000 topographic data, we present an improved map of the active Red River fault zone between Midu (Yunnan, China) and Hanoi (Vietnam). The fault zone is composed of parallel strands, one of which, the Yuanjiang fault was previously undetected. There also appears to be a component of extension all along the fault zone. Such extension increases toward the SE, from Yunnan to the south China sea coast, and the vector describing the motion of south China relative to Indochina points within the N45°–135°E quadrant. We attempt to assess the Plio-Quaternary dextral slip rate on the Red River fault (RRF) by restoring large river offsets and searching for the largest, plausible one. Across much of Yunnan, the fault is perpendicular to local catchments that drain into the Red River. From precise mapping of the river courses on SPOT satellite images and on 1/100,000 topographic maps, numerous multiple offsets along the fault can be detected and reconstructed. The lack of correlation between the apparent offsets and the lengths of the rivers upstream from the fault suggests either that the drainage system was in large part established prior to the onset of dextral slip along the fault or that frequent captures have occurred. We thus try to find the best fit between series of river channels upstream and downstream from the fault by progressively restoring the dextral displacement in increments of 500 m, up to an offset of 50 km. For each increment we measure the misfits (root mean squares, RMS) between the upstream and downstream channels. The best fit and smallest RMS are obtained for an offset of 25±0.5 km that we interpret to represent the clearest, large right-lateral displacement recorded in the geomorphology along the active Red River fault. Since dextral motion is likely to have started around 5 Myr, the most probable average Plio-Quaternary slip rate on the fault is of order of 5 mm/yr. We attribute the apparent lack of seismic activity on a large stretch of the fault to millennial recurrence times between great earthquakes. Our study shows that relatively small drainage systems can keep a good record of fairly large cumulative fault offsets.

An Early Miocene Transition in deformation regime within the Red River Fault Zone, Yunnan, And its significance for Indo-Asian tectonics

Recent work has tended to support the view that lateral extrusion of Indochina along the Red River fault zone was a significant factor in accommodating the convergence of India with Asia. To better understand the nature and timing of this feature, 40Ar/39 Ar age spectrum analyses were undertaken on minerals from the Ailao Shan/Red River metamorphic belt in southern Yunnan, China. Interpreted using diffusion domain theory, these results yield thermal histories in the temperature range 550–150°C. Discontinuities in cooling rates observed from these results suggest a transition in deformation style in the shear zone during the early Miocene. At least in the vicinity of our samples, left-lateral, strike-slip ductile deformation appears to have ceased by about 20 Ma. The mylonitic gneisses in the shear zone were subsequently obliquely unroofed, probably due to a component of normal faulting along the eastern edge of the shear zone causing a brief (locally 100°C/m.y.) cooling episode. This transition may have resulted from a component of northeast-southwest extension attributable to greater clockwise rotation of Indochina relative to South China after ∼20 Ma than that consistent with pure strike-slip along the fault. K-Ar ages on clay minerals from gouge within the normal faults bounding the Ailao Shan and Diancang Shan yield a range of ages between 20 and 185 Ma but show a significant clustering at 20–25 Ma. These results suggest that early Miocene movement on a precursor to the Range Front normal fault in the Ailao Shan caused oblique unroofing of the shear zone. This early Miocene event corresponds in time with a transition from low to very high denudation rates throughout the Himalaya and southern Tibet. This coincidence may reflect that once left-lateral strike-slip motion on the Red River fault zone slowed or was terminated, accommodation of much of the continued northward convergence of India occurred on the Main Central Thrust resulting in rapid uplift in the Tethyan Himalaya and Gangdese belt.

New U-Th/Pb constraints on timing of shearing and long-term slip-rate on the Karakorum fault

[1] Zircons and monazites from 6 samples of the North Ayilari dextral shear zone (NAsz), part of the Karakorum fault zone (KFZ), have been dated with the U-Th-Pb method, using both ID-TIMS and SIMS techniques. The ages reveal (1) inheritance from several events spanning a long period between the late Archean and the Jurassic; (2) an Eocene-Oligocene magmatic event (similar to 35-32 Ma); (3) an Oligo-Miocene magmatic event (similar to 25-22 Ma), at least partly synkinematic to the right-lateral deformation; and (4) a period of metamorphism metasomatism (similar to 22-14 Ma) interpreted as thermal and fluid advection in the shear zone. The Labhar Kangri granite located similar to 375 km farther Southeast along the KFZ is dated at 21.1 +/- 0.3 Ma. Such occurrence of several Oligo-Miocene granites along the KFZ, some of which show evidence for synkinematic emplacement, suggests that the fault zone played an important role in the genesis and /or collection of crustal melts. We discuss several scenarios for the onset and propagation of the KFZ, and offset estimates based on the main sutures zones. Our preferred scenario is an Oligo-Miocene initiation of the fault close to the NA range, and propagation along most of its length prior to similar to 19 Ma. In its southern half, the averaged long-term fault-rate of the KFZ is greater than 8 to 10 mm/a, in good agreement with some shorter-term estimates based on the Indus river course, or Quaternary moraines and geodesy. Our results show the KFZ cannot be considered as a small transient fault but played a major role in the collision history.

Discussion on the role of the Red River shear zone, Yunnan and Vietnam, in the continental extrusion of SE Asia Journal, Vol. 163, 2006, 1025–1036

P. H. Leloup, P. Tapponnier & R. Lacassin write: In his recent paper, Searle (2006) acknowledges that the 1000 km long Ailao Shan–Red River shear zone is a large Miocene left-lateral shear zone, but speculates that left-lateral slip started after 21 Ma and claims that the total finite offset remains unknown. From this he concludes that continental extrusion was only a relatively minor tectonic factor during the India–Asia collision, as long argued by other workers (e.g. England & Houseman 1986; Cobbold & Davy 1988; Dewey et al . 1989; Houseman & England 1993). We summarize below the field and geochronological evidence that makes us maintain a viewpoint in better accordance with facts. Timing of left-lateral shear along the Ailao Shan–Red River shear zone . The Ailao Shan–Red River shear zone is composed mostly of high-grade metamorphic rocks and deformed granitoids with ubiquitous evidence for left-lateral shear parallel to the belt (e.g. Tapponnier et al . 1986, 1990; Leloup et al . 1993, 1995, 2001). The crystallization of the granitoids has been dated between 22 and 35 Ma (Fig. 1d–f), for example by Scharer et al . (1990, 1994) and Zhang & Scharer (1999), leading those workers to propose that left-lateral shear started at least c . 35 Ma ago. In contrast, Searle (2006) claims that all the deformed granitoids found within the shear zone predate left-lateral shear and that their crystallization ages should thus be interpreted to provide an upper limit for the onset of deformation, rather than a lower limit, thus suggesting a maximum age of 21 Ma for this deformation. A clear understanding of the P – T history and in situ deformation history of the intrusions, as well as of their relationships with surrounding paragneisses, is fundamental for interpreting correctly the geochronological …

Direct measurement of strain rates in ductile shear zones: A new method based on syntectonic dikes

Received 17 January 2008; revised 31 August 2008; accepted 29 October 2008; published 23 January 2009. [1] We describe a new method to estimate directly ductile strain rates at an outcrop scale from the deformation of dikes emplaced within a shear zone. The method is tested in a well-constrained shear zone: the Ailao Shan–Red River shear zone, for which global strain rates can be calculated from published fault rates. The strain rate was determined by measuring independently the shear strain (g) recorded by the dikes and the age (t) of dikes emplacement. The shear strain was quantified by three different methods that take into account either the stretching of the dikes or their angle variations during deformation or both of them. The values of minimum shear strains range between 0.2 and 9.7 for the less to the most deformed dikes, respectively. The ages of dike emplacement were obtained by Th-Pb sensitive high-resolution ion microprobe (SHRIMP) dating of monazites. We obtained three groups of ages: the younger age is 22.55 ± 0.25 Ma, the intermediate age is 26.81 ± 0.66 Ma, and the oldest ages are 29.89 ± 0.46 Ma and 29.93 ± 0.38 Ma. The geochronological data are in agreement with the structural data, the most deformed dikes being the oldest. The minimum strain rates deduced from these measurements are 3 to 4 � 10 � 14 s � 1 , which is consistent with previous estimates of geological strain rates in ductile shear zones.

Ductile strain rate measurements document long-term strain localization in the continental crust

Quantifi cation of strain localization in the continental lithosphere is hindered by the lack of reliable deformation rate measurements in the deep crust. Quartz-strain-rate-metry (QSR) is a convenient tool for performing such measurements once calibrated. We achieve this calibra- tion by identifying the best piezometer-rheological law pairs that yield a strain rate in agree- ment with that measured on the same outcrop by a more direct method taken as a reference. When applied to two major continental strike-slip shear zones, the Ailao Shan-Red River (ASRR; southwest China) and the Karakorum (northwest India), the calibrated QSR high- lights across-strike strain rate variations, from 1 ◊ 10 −13 s -1 in zones where it is localized. Strain rates integrated across the shear zones imply fast fault slip rates on the order of 1.1 cm yr -1 (Karakorum) and 4 cm yr -1 (ASRR), proving strong strain localization in these strike-slip continental shear zones.

Timing and rate of exhumation along the Litang fault system, implication for fault reorganization in Southeast Tibet

The Litang fault system that crosses the Litang Plateau, a low relief surface at high elevation (~4200–4800 m above sea level) that is not affected by regional incision, provides the opportunity to study exhumation related to tectonics in the SE Tibetan Plateau independently of regional erosion. Combining apatite and zircon fission track with apatite (U-Th)/He thermochronologic data, we constrain the cooling history of the Litang fault system footwall along two transects. Apatite fission track ages range from 4 to 16 Ma, AHe ages from 2 to 6 Ma, and one zircon fission track age is ~99 Ma. These data imply a tectonic quiet period sustained since at least 100 Ma with a slow denudation rate of ~0.03 km/Ma, interrupted at 7 to 5 Ma by exhumation at a rate between 0.59 and 0.99 km/Ma. We relate that faster exhumation to the onset of motion along the left-lateral/normal Litang fault system. That onset is linked to a Lower Miocene important kinematic reorganization between the Xianshuihe and the Red River faults, with the eastward propagation of the Xianshuihe fault along the Xiaojiang fault system and the formation of the Zhongdian fault. Such strike-slip faults allow the sliding to the east of a wide continental block, with the Litang fault system accommodating differential motion between rigid blocks. The regional evolution appears to be guided by the strike-slip faults, with different phases of deformation, which appears more in agreement with an “hidden plate-tectonic” model rather than with a “lower channel flow” model.

New constraints on the timing of partial melting and deformation along the Nyalam section (central Himalaya): implications for extrusion models

Abstract New structural, U–Th/Pb and Ar/Ar data along the Nyalam section constrain the timing of partial melting, crystallization and deformation in the Greater Himalayan Sequence. Prograde metamorphism was followed by the onset of partial melting at c. 30 Ma. In the central Greater Himalayan Sequence, in situ melts crystallized between 24 and 18 Ma. Subsequent cooling was very fast (c. 200 °C Ma−1) and coeval with the emplacement of undeformed dykes that lasted until c. 15 Ma. In the upper Greater Himalayan Sequence, fast cooling continued until c. 13 Ma. Combined with published P–T and thermochronological data from the Langtang and Dudh Kosi valleys, these data imply that: (a) the partial melt zone thinned over time; (b) the end of melting preceded the end of motion on the Main Central Thrust and the South Tibetan Detachment by 6 and 2 Ma, respectively; (c) the South Tibetan Detachment possibly initiated at c. 25 Ma, probably reactivating a pre-existing thrust; and (d) the present-day topography has been established for <6 Ma and focused erosion on the present-day southern slopes of the Himalaya was not active at the time of the exhumation of the Greater Himalayan Sequence. These observations suggest that the Main Central Thrust/South Tibetan Detachment systems are not passive structures induced by focused erosion, as has been suggested previously by some lower crustal channel flow models. Supplementary material: U/Pb, geochemistry and Ar/Ar data, the Ar/Ar analytical procedure, field pictures, T11N26, T11N27, T11N31, T11N53A and T11N40 quartz ⟨C⟩ axis CPOs are available at http://www.geolsoc.org.uk/SUP18835